The need for radiation dosimeters, especially of the type to be carried about by individuals, is well recognized. The most popular of the devices used at the present time is the photographic film badge in which the amount of radiation is determined by the number of tracks.
The track damage dosimeter is a device in which a very small track is left in the material (e.g. mica, polycarbonate) when hit by radiation. To make the track large enough to measure, an etching solution is applied, after which the number of tracks can be counted. The device, therefore, is indirect reading. It is not sensitive to gammas; it is sensitive to neutrons. To make this device work, fissionable radiation foils must be used that is, the wearer is exposed to radiation by the device which is designed to measure radiation. The exposure in one of these devices, despite a 2 mm lead shield, was 50 millirad per hour in the area under the badge. Such devices are indirect reading; also, they cannot readily be adjusted with respect to the minimum energy, i.e. threshold energy, to which they will respond.
Also available are devices which employ electrometers which are sensitive to gammas and/or both neutrons and gammas. These devices are direct reading, but must be charged before use. They are fairly sensitive to vibration. (The electrometer may lose its charge thereby giving a false radiation reading). Devices of this type can be purchased for between $50.00 and $150.00 and thereafter need only be charged on-site so that there are no recurrent costs.
The thermoluminescent detector, known as TLD, is a solid state device in which the active element, after exposure to nuclear radiation, will luminesce on being heated. The integrated light flux produced is a measure of the radiation of a specific type to which the TLD has been exposed. By selection of appropriate materials, the TLD can be made to distinguish between different types of radiation. However, just as is the case with film badges, the TLD cannot be read directly, although on-site devices are available for reading the TLD.
The principle of radiation-induced nucleation of super-heated liquids has been used in the bubble chamber; such a device has a relatively large volume of liquid under pressure. At the appropriate time the pressure is quickly dropped, thereby placing the liquid in the superheated state, radiation being incident on the sample. Bubbles form along the path of radiation under properly controlled conditions. A serious disadvantage is that the bubble chamber can be kept at the low pressure only for very brief periods of time before re-applying higher pressure, because bubble nucleation begins where it is not desired very quickly (on container surfaces and in the bulk).
Another device depending on superheated liquids is the ultrasonic bubble chamber. Like the regular bubble chamber, this device is used for tracking radiation. However, here the pressure is rapidly cycled acoustically e.g. an acoustic standing wave can produce regions where the pressure oscillates locally. In these regions a vapor bubble may form as radiation passes if the pressure has dropped sufficiently below the ambient pressure.
Skripov* has studied droplets of liquids which rise in a heated host liquid. At a certain point the droplets become superheated. The higher they rise, the more they become superheated. Eventually the droplets will vaporize at the "limit-of-superheat" for the given pressure. But if exposed to gammas, they will vaporize before they reach their "limit-of-superheat" temperature. Skripov has stopped these droplets by intercepting them with a glass dish. As long as a little of host liquid remains between the glass and the stopped droplet, the droplet will not vaporize prematurely. FNT *V. P. Skripov, Metastable Liquids. Halsted Press, John Wiley & Sons, New York, 1974 (Translation). Russian book published 1972.
As is evident, Skripov's research tool is not a practical device; it employs a single droplet or perhaps a small number, but not many; it does not integrate the effect over time because the vapor is not retained; the host material in many of Skripov's experiments is sulfuric acid -- which is certainly an impractical liquid for personnel dosimetry.
Skripov and his colleagues also used capillary tubes filled with sensitive liquid material which could be superheated by dropping the pressure (same reference). These were exposed to radiation sources, and the time before vaporization could be measured. The capillary tube device is far less stable than the droplet method, i.e., the lifetime of a superheated liquid in a capillary tube is relatively short because of the large surface area in contact with glass (or quartz) and the large volume of liquid used. Heterogeneous nucleation, i.e. nucleation caused by solid particles or contact with walls, will cause vaporization of the liquid, making capillary tubes an impractical device as a radiation detector.
An acoustic field in a liquid produces an oscillating pressure field, thereby sensitizing (i.e. superheating) and desensitizing the liquid at the acoustic frequency. Also the high acoustic fields can be generated locally, away from container surfaces, thereby sensitizing only a relatively small volume. The interaction of radiation with liquid is noted by an acoustic cavitation event, -- a vapor bubble (or bubbles) grows and collapses, making an audible snapping sound, which can be recorded. Unfortunately, heterogeneous cavitation can occur unless the liquid is very carefully filtered. Also, a cavitation event may provide heterogeneous nucleation sites, i.e. the liquid must be refiltered before achieving an acceptably clean liquid. However, it is not a practical device for monitoring because of the down time in between cavitation events.
As is evident then, although a variety of devices based on several different principles have been used for personnel monitoring, none of these has proved completely satisfactory. What is needed is a small, inexpensive device which is reliable and which can be monitored as frequently as desired without terminating the usefulness of the specific instrument as is the case when the film of a film badge is developed.